The present invention relates to a radio communication system in which the quality of propagation paths varies over time, and more particularly, to a modulation method adapted to the varying quality of propagation paths and a method of controlling the coding rate of a channel code.
For increasing the amount of information transmitted per unit time in a radio communication system, multi-level modulation techniques are known for transmitting a plurality of bits of information per symbol.
In the multi-level modulation techniques, a larger number of bits per symbol results in a higher maximum throughput on propagation paths of higher quality, whereas errors are more likely to occur when the propagation path quality degrades, giving rise to a problem of a largely deteriorated throughput. For this reason, adaptive modulation techniques have been proposed in order to make stable communications in accordance with the propagation path quality. Specifically, the adaptive modulation techniques involve switching one modulation type to another such that a larger number of bits are modulated per symbol when a transmission path presents a high quality, while a smaller number of bits are modulated per symbol when a transmission path presents a low quality. Such techniques are described in an article entitled “Performance of Modulation Level Controlled Adaptive Modulation Systems” (The Institute of Electronics, Information and Communication Engineers Transaction B-II, Vol. J78-B-II, No. 6, pp. 435-555, June 1995).
Also, techniques have also been proposed for making communications at a throughput suitable for a particular quality of a propagation path by switching an coding rate of channel codes in addition to a quality-based modulation, and has been used, for example, in a system conforming to the standard “cdma2000 High Rate Packet Data Air Interface Specification” (3GPP2 C.S0024-A Version 1.0, March, 2004). Further, in the system described in “cdma2000 High Rate Packet Data Air Interface Specification,” a transmitter station previously encodes a signal at a low coding rate, and transmits part of the encoded signal which is then decoded at a receiver station. When the signal is successfully decoded, the transmission is terminated. On the other hand, when the receiver station fails to demodulate the signal, the transmitter station transmits another part of the encoded signal again, and the receiver station decodes the other part of the signal in combination with the previously received signal, thereby adjusting the coding rate in accordance with an actual propagation path quality to make communications at a throughput suitable for the propagation path. This strategy is called HARQ (Hybrid Automatic Repeat reQuest).
Since a large amount of processing is generally required for decoding channel codes, a receiver station in the aforementioned HARQ scheme needs to decode an encoded signal each time the receiver station receives part of the encoded signal. It is therefore desirable to enhance the processing capability required for the receiver station. Also, in the HARQ scheme, the transmitter station must determine whether a retransmission should be done after the receiver station has completed the decoding, thereby causing a longer interval of transmission at the transmitter station, and a resulting increase in signal delay. It is therefore desirable to increase the capacity of a storage device required for holding largely delayed signals.